Saved Imei in iPhone
Where it is saved in the iPHone IMEI?
苹果官方于2015年9月25日正式发售iPhone6S A1688/A1633机型。 此机型的修理方式与先前几代差不多, 需要螺丝起子及撬棍工具。 同时也适用GSM or CDMA/16, 64 or 123 GB/ 银色, 金色, 深空灰色, 玫瑰金色等机型。
You can't, period.
Apple is a group of control freaks, they paired and encrypted almost every chip that has logic functions. So replacing just some of the chips will destroy the device.
For example in order to replace IMEI, the baseband and baseband flash ROM chip must be replaced together as a set, or the device dies. Also the device S/N must be rewritten to pair with the IMEI, or the phone will be denied activation. Rewriting S/N is not yet possible for iPhone 6s, so just forget about replacing chips.
If you replace the logic board, you'll run into another problem. The Touch ID sensor is paired with the CPU, Touch ID dies immediately when you replace just the board, and if you attempt to reinstall iOS, jackpot, the phone dies.